A. Field of the Invention
This invention relates to a recording medium used in recording devices, particularly hard disk devices, mounted in consumer equipment or in information recording devices of computers or other information processing equipment.
B. Description of the Related Art
Increases in the quantity of information handled by computers and other information processing equipment in recent years, and miniaturization of information recording equipment, have been accompanied by expansion of the recording capacities of information recording devices, and recording capacities sought from magnetic recording media used in information recording devices continue to trend upward. In order to increase the recording capacities and improve the recording performance of magnetic recording media, the distance between the read/write elements of the magnetic head and the magnetic layer of the magnetic recording medium, that is, the magnetic spacing, must be decreased to the maximum extent possible. The magnetic spacing is determined by the thickness of the magnetic head protective layer, the flying height of the magnetic head, and the thicknesses of the protective layer and lubrication layer of the recording medium. One development goal for recording media is reduction of the thickness of the protective layer. As the protective layer for recording media, an amorphous carbon material called DLC (Diamond-Like Carbon) is generally employed.
FIG. 2 is a schematic cross-sectional view of a recording medium of the prior art. In FIG. 2, recording medium 1 comprises substrate 2, magnetic layer (magnetic metal layer) 3 in which information is recorded by a magnetic head, protective layer 4 which protects magnetic layer 3 from corrosion, wear, shocks, and other damage, and lubrication layer 5 covering the surface of protective layer 4. Substrate 2 is formed from glass, an aluminum material with a plated surface, or similar, and serves as a base. Magnetic layer 3 is a granular layer comprising, for example, Co, Cr, Pt, or another metal, and SiO2 or similar, and is deposited by sputtering. It is preferable that DLC be employed as protective layer 4. The protective layer is deposited by sputtering or by plasma CVD (Chemical Vapor Deposition). PFPE (perfluoropolyether) is preferably used in lubrication layer 5, which is formed to a thickness of approximately 1 nm by a dipping method or similar. The recording density of a recording medium of the prior art is approximately 500 Gbits/in2, and the thickness of protective layer 4 is approximately 3 nm. Hereafter, in order to raise the recording density to 750 Gbits/in2 or higher, the thickness of protective layer 4 must be reduced to 2 nm or less. To further raise the recording density to 2000 Gbits/in2 in the future, the thickness of protective layer 4 must be made approximately 1 nm.
On the other hand, protective layer 4 of a recording medium is also required to have sufficient reliability, that is, corrosion resistance, sliding durability, and head flying characteristics, and similar reliability is required even when the film thickness is reduced. However, adequate reliability, and in particular corrosion resistance and sliding durability, has not been obtained from DLC formed by methods of the prior art in the thickness range of 2 nm or less. It is thought that by forming protective layer 4 with a finer texture, that is, by increasing the sp3 ratio of the carbon layer, adequate reliability may be obtained.
In order to raise the sp3 ratio of protective layer 4, sufficient energy may be imparted to particles obtained by dissociation and ionization of carbon raw material in a process of subplantation, that is, causing the particles to penetrate below the surface layer. The penetrating ionized particles enter a state of high compressive stress in the carbon layer (protective layer 4), and as a result carbon sp3 bonds are induced, and the sp3 ratio is raised. Specifically, a method called the FCA (Filtered Cathodic Arc) or FCVA (Filtered Cathodic Vacuum Arc) method is known. In this method, arc discharge is used to generate plasma from a graphite or other carbon target, and film is deposited by means of the C+ and other ionized particles contained therein, raising the sp3 ratio, and at the same time enabling formation of a hard carbon layer not containing hydrogen. FCA has been used to fabricate protective layers 4 of magnetic heads in hard disk devices, and its application to recording media has been studied.
However, at the beginning of film deposition, that is, in the stage of this method in which film deposition on the magnetic layer 3 is begun, ionized particles directly penetrate magnetic layer 3, and metal atoms with which the ionized particles collide recoil in the carbon layer direction, so that mixing with carbon atoms occurs. A carbon layer in such a mixed state cannot have the characteristics inherent to carbon; that is, it can be said that the initial growth layer has a low sp3 ratio. Further, a metal element which is a cause of corrosion is included, and this also leads to degradation of corrosion resistance. If protective layer 4 is sufficiently thick no problems arise, but if protective layer 4 is thin, the thickness of the initial growth layer cannot be ignored, and consequently adequate corrosion resistance cannot be obtained.
One report of measures to address, mixing is given in Nobuto Yasui, Hiroshi Inaba and Naoto Ohtake: Applied Physics Express 1 (2008), 035002, in which an underlayer comprising a silicon material is interposed between a carbon layer formed by FCA and a metal layer, preventing penetration of ionized particles into the metal layer and preventing recoil of the metal element, to improve corrosion resistance.
As other proposals to apply an underlayer to a carbon layer similarly to that described above, there exist Japanese Patent Application Laid-open No. H9-138943 and Japanese Patent Application Laid-open No. H11-203625. In Japanese Patent Application Laid-open No. H9-138943, in addition to a silicon material, underlayers of germanium, tin, and other materials are interposed, and these are used as buffer layers to reduce the residual strain in the carbon layer. By this technique sliding durability is improved, and the protective layer can be made thinner, but the protective layer thickness is still 2.5 nm, and there is further room for improvement. Japanese Patent Application Laid-open No. H11-203625 is an example of application to a magnetic head. A material having silicon as its main component is interposed as an underlayer, with the underlayer thickness at 0.5 nm or greater and the carbon layer thickness at 2 nm or greater. The underlayer functions as an adhesive layer, and it is claimed that by this means adhesion of the carbon layer is enhanced, sliding durability is improved, and the protective layer thickness can be reduced. However, in this method the surface on which the protective film is formed is polished to reduce the roughness and obtain a protective film with uniform coverage, and application to a recording medium is difficult. Further, it is stated that at protective layer thicknesses, that is, total thicknesses for the carbon layer and underlayer of less than 2 nm, the film is not continuous but has an island-like morphology, and reliability is insufficient. Further, in Japanese Patent Application Laid-open No. H11-203625, CCP (Capacitively Coupled Plasma)-CVD and ECR (Electron Cyclotron Resonance)-CVD methods are used as the method of formation of the protective layer, and a protective film into which sp3 bonds are appropriately introduced cannot be formed.
It has also been proposed that reliability be improved by regulating the hydrogen content of the carbon layer, to accommodate reduced thickness of the protective layer. In Japanese Patent Application Laid-open No. 2008-123671, the hydrogen content of the carbon layer is limited to from 8 to 18 at %, to raise the sp3 ratio and improve the medium reliability. In Japanese Patent Application Laid-open No. 2004-152462, the hydrogen content of the carbon layer surface is made 30% or lower, to secure head flying characteristics and improve reliability. The hydrogen contents in both references are low values for DLC, and lowering the amount of hydrogen leads to improvement. The previously described reference of Nobuto Yasui, Hiroshi Inaba and Naoto Ohtake: Applied Physics Express 1 (2008), 035002 uses the FCA method of film deposition, in which hydrogen is essentially not included in the film, and thus can be said to use the same approach.
While the above-described proposals have effects in improving reliability for protective layer thicknesses in a certain range, at protective layer thicknesses which are further reduced, reliability has been inadequate.
The present invention was devised in light of the above-described problems, and provides a recording medium for which corrosion resistance, sliding durability and head flying characteristics can be secured, and which enables reduced magnetic spacing while securing reliability, so that recording densities of for example from 750 to 2000 Gbit/in2 can be accommodated.
In order to resolve the above problems, a recording medium of this invention is used to record and reproduce information by means of a head which performs information readout and writing based on magnetic principles. The medium comprises a magnetic layer formed on a substrate and a protective layer formed on the magnetic layer. The protective layer comprises an underlayer formed on the magnetic layer and including a material selected from the group consisting of silicon, silicon carbide and germanium, and a carbon layer formed on the underlayer and including amorphous carbon containing hydrogen; an amount of hydrogen included in the carbon layer is 24.7 at % or higher and 46.8 at % or lower, a thickness of the underlayer is 0.3 nm or greater and 1.8 nm or less, and a thickness of the carbon layer is 0.2 nm or greater and 1.7 nm or less.
It is preferable that the hydrogen amount included in the carbon layer exceed 30.3 at % and be 46.8 at % or lower, and that a total thickness of the underlayer and the carbon layer be 1 nm or greater and 2 nm or less.